1. Field of the Invention
The present invention relates to a fail control apparatus for a vehicular automatic transmission, and more particularly to a fail control apparatus which provides control in the event of a failure of a valve or the like which controls operating oil pressure applied to a friction element.
2. Description of the Prior
Conventionally, a control apparatus for a vehicular automatic transmission detects an electric failure of a solenoid, a sensor, or the like in a hydraulic control circuit and provides control to cope with the failure.
Examples of failures occurring in automatic transmissions, however, include not only the electric failure mentioned above but also a malfunction of a valve which controls operating oil pressure applied to a friction element when the valving element of the valve sticks or a foreign matter is caught in the valve.
If, due to such a valve malfunction, a friction element which should be engaged is disengaged or a friction element which should be disengaged remains engaged, a phenomenon such as shifting to a gear position different from a designated gear position or a neutral failure, i.e. shifting to neutral occurs, making it impossible for the vehicle to start moving again from a parked position.
The occurrence of such a failure can be detected with relative ease, but it is difficult to determine which friction element is concerned with the failure as distinct from the case of an electric failure.
To address this problem, it has been proposed in, for example, Japanese Laid-Open Patent Publication (Kokai) No. 2000-240785 that one friction element which has failed is identified by comparing the gear ratio of a designated gear position and the actual gear ratio, and control is carried out according to the failure of the identified friction element.
With this technique, when engagement caused by failure occurs, i.e. a friction element remains engaged although it should be disengaged, shifting to a gear position which is achieved by engaging the friction element engaged due to failure is carried out, and conversely, when disengagement caused by failure occurs, i.e. a friction element remains disengaged although it should be engaged, shifting to a gear position which is achieved without engaging the friction element disengaged due to failure is carried out.
According to the above conventional technique, however, a friction element which has failed is identified by comparing the gear ratio of a designated gear position and the actual gear ratio with respect to all the gear positions without exception during driving, and hence in actuality, there is the possibility of erroneous determination.
For example, in an automatic transmission described in Japanese Laid-Open Patent Publication (Kokai) No. 2000-240785, the second speed gear position is achieved by engaging a low clutch and a 2&4 brake among friction elements. In a case where a designated gear position at the time of failure determination is the second speed gear position, it is estimated that the low clutch is disengaged due to failure or the 2&4 brake is disengaged due to failure.
According to an engagement table of this automatic transmission, shifting to neutral (N) occurs when the low clutch is disengaged due to failure, and shifting to the first speed gear position occurs when the 2&4 brake is disengaged due to failure.
However, when either of these two failures occurs while the vehicle is moving at a high speed, the actual gear ratio cannot be determined since the speed of an engine cannot be accelerated to the first revolutionary speed corresponding to the first speed gear position. Also, depending on the driving conditions, engine torque may peak out at a gear ratio corresponding to the first speed gear position, and it may be erroneously determined that the 2&4 brake has been disengaged due to failure although in actuality, the low clutch has been disengaged due to failure.
Since determining the gear ratio and identifying a failed part take a lot of time, the above conventional technique is not effective for quickly coping with a failure.